The invention relates in general to focused ion beam processing and more particularly to the repair of a workpiece, having an opaque film patterned on a substrate, such as a photomask, X-ray mask, or reticle.
Manufacturers of such workpieces are extensively employing focused ion beam (FIB) methods for the selective removal of material without the use of a patterned resist mask. Advantageously, a FIB system can operate both as a scanning ion microscope (SIM) and as a precision milling system. Thus, using FIB methods, manufacturers can image a workpiece (deriving the image from ion beam induced particles, e.g. secondary-electrons or secondary-ions), locate a defect on a patterned opaque film of a workpiece, and then mill micron or submicron scale features at the location of the defect. The term manufacturers, as used herein, encompasses both those who create and those who repair the above-mentioned workpieces.
Thus, one of the primary applications for FIB micromachining systems is the repair of masks and reticles. In many applications, manufacturers employ pure sputtering, i.e. not gas-assisted sputtering, to mill the desired structures. Scanning a focused beam of ions over a substrate surface physically sputters away substrate particles, including atoms, ions and molecules. Non-volatile sputtered material will condense on any surface it encounters. This effect, known as redeposition, limits the precision of microstructure fabrication. The redeposition effect is particularly noticeable at the side walls of an etched recess, especially if the recess has a high aspect ratio, e.g. a narrow, deep groove.
Some shortcomings presently found in the FIB repair of defects in a patterned film on a substrate are:
incomplete removal of opaque material PA1 a decrease in the transmission of electromagnetic radiation due to absorption by ions implanted in the substrate (normally quartz). This "staining" phenomenon is more pronounced as the lithographic illumination moves from near ultraviolet, i.e. UV (365 nm), to deep ultraviolet, i.e. DUV (248 nm and 193 nm), wavelengths; and PA1 excess removal of substrate below and surrounding (riverbed effect) the opaque defect.
During particle beam processes, such as particle beam deposition and particle beam etching with a focused beam, the workpiece to be processed is disposed within a vacuum chamber and positioned beneath a column that generates a particle beam. The particle beam column is activated and generates particles that strike the surface of the workpiece. To facilitate the processing of the workpiece, reactant materials, typically fluids, and more typically gases, can be directed at the surface of the workpiece being processed. The reactant materials cooperate with the particle beam to enhance or modify the deposition or etching process being performed. When a gas is directed at the surface of the workpiece during FIB etching, the process is typically referred to as gas-assisted etching (GAE).
U.S. Pat. No. 4,951,097 by Hattori et al., incorporated herein by reference, discloses an apparatus for repairing a pattern film using a Chlorine etching gas. However, a GAE system that uses chlorine has drawbacks. An effective chlorine GAE system requires a vacuum pump that tends not to generate impurity gas. Also, the GAE system itself is subject to the corrosive effect of the chlorine. Furthermore, Chlorine may not provide selective etching or enhanced etching to the extent required by current manufacturers.
Japanese Patent Application No. 6-129260, incorporated herein by reference, discloses using Iodine gas during GAE. However, Iodine also has disadvantages. Iodine often requires heating to establish enough vapor pressure to assist in the etching process. Heated elements within a chamber contribute to thermally-induced mechanical drift of the assembly holding the mask, which decreases an operator's ability to maintain the mask's location relative to the FIB over time. This heating can also cause thermal expansion of the mask. Such thermal expansion of the mask during FIB micromachining is undesirable because the dimensions of the mask's microstructures are critical. Iodine can also be difficult to pump off the workpiece. Thus, iodine can continue to etch when the workpiece is removed from the vacuum chamber. Furthermore, Iodine can be difficult to use because of its odor.
GAE is generally described in "Characteristics of gas-assisted focused ion beam etching" by R. J. Young, J. R. A. Cleaver, and H. Ahmed, J. Vac. Sci. Technol. B., 11(2), p.234, (1992), incorporated herein by reference.
Accordingly, it is an object of this invention to provide methods of GAE that provide an improved repair of defects in an opaque film patterned on a substrate.